One or more power generation systems, such as wind turbines, may be connected to a utility grid to supply electric energy to the utility grid. The power provided by the wind turbine generator and the power needed by the utility grid may have different frequencies, in particular electrical frequencies. In a wind turbine, a frequency converter is used to interface the AC (alternating current) power from the generator to the AC power of the utility grid.
There may be a situation where a wind farm is producing power into the electrical system, i.e., the utility grid, and part of this electrical system becomes islanded, i.e., disconnected from the rest of the grid. There may be also a situation where a wind farm is producing power into one end (the non grid connected end) of an HVDC scheme, and either the electrical system becomes islanded, or the HVDC scheme blocks, meaning the switching devices within the remote HVDC power converter terminal are turned off.
In the case of an islanded electrical system, a wind farm may be producing power into the electrical system and the part of this electrical system to which the wind farm is connected may become islanded, with undefined local loads, and undefined local generation on the Islanded system.
In the event of an islanded electrical system, a common vector current controller algorithm will ‘track’ the frequency and voltage of the islanded system and unless the load and generation are perfectly balanced, the islanded system frequency will deviate from ‘nominal’ and eventually deviate by a sufficient degree that the power converter (and/or other turbine or electrical system components) trip or shut down. A common network bridge current controller does nothing to actually help maintain ‘nominal’ voltage or frequency, it does not act in the same manner as a synchronous generator.
Thus, there may be a need for a controller being adapted to be controlled during conditions when a power converter (within for example a wind turbine) should be controllable in such a manner to specifically support or maintain the nominal voltage and frequency on an islanded system.